Inductive power transfer (IPT) or inductively coupled power transfer (ICPT) systems are well known and used for a number of industrial applications, and have particular advantages where traditional methods are unable to perform satisfactorily, for example clean rooms, people moving, materials handling, battery charging, or any application requiring a substantially contactless supply of power.
A typical IPT system consists of three main components; an AC power supply, a primary conductive path, and one or more electrically isolated pickups coupled with a load and provided substantially adjacent the primary conductive path. Together, the power supply and primary conductive path form the primary side of an inductive power transfer system, while the pickup and associated circuitry forms the secondary side.
The primary conductive path, typically in the form of an elongated conductive loop or track, is energised by the AC power supply to create a continuously varying magnetic field about the track. The or each pickup includes an inductive coil, in which a voltage is induced by the changing magnetic flux passing through the coil in accordance with Faraday's law of induction, thereby achieving contactless inductive power transfer.
Typically, the or each pickup will include some form of controller circuit to control the transfer of power to the load, including a switched-mode controller such as a boost converter, for example, to supply the relatively constant output voltage required by the load, thereby providing secondary-side output power control.
In some circumstances, it may be preferable to control the output power of the pickup from the primary side of the system. This may be due to the additional cost and complexity of the additional components required for a pickup side controller or due to limitations on the physical size of the pickup, for example. Power supplied to a load associated with a pickup may be controlled from the primary side of an IPT system by modulating the amplitude, phase or frequency of the current in the primary conductive path to reduce power at partial loads and minimise losses, or to increase the current in the primary conductive path and therefore the magnetic flux when the pickup coil is not ideally aligned, for example, to ensure the required power is supplied to the load. Providing primary-side power control can help reduce costs by eliminating the power conditioner and switched-mode controller from the pickup circuit.
However, although primary-side power control may be preferred in some circumstances, there are disadvantages in the primary-side power control methods of the prior art. For example, frequency modulation, wherein the frequency of the current in the primary conductive path is varied with respect to the resonant frequency of an inductively coupled pickup circuit to tune and/or detune the resonant circuit, is very difficult to achieve in practice and requires a complex power controller. A power controller which varies the current in the primary conductive path or track by amplitude modulation can be effective. However, in both cases, the system requires some form or wired or wireless communications to form a feedback loop between the pickup output on the secondary side of the system, and the power supply on the primary side. This adds to the complexity and cost of the system.